1. Field of the Invention
The invention relates to telephone overload protection circuits and particularly to a circuit arrangement for protecting a telephone subscriber line feed circuit in a preferably digital telephone exchange system from overloads occuring due to a short-circuit or an impermissibly low-ohmic shunt between the wires of the subscriber line.
2. Description of the Prior Art
In view of the necessity to feed power to a telephone line, the feed resistors in subscriber feed lines are relatively low-ohmic. The currents which flow through these feed resistors in the case of a short-circuit or in the case of an impermissibly low-ohmic shunt are sufficiently substantial to cause the power consumption which occurs across the feed resistors to lead to their damage.
In a known circuit arrangement, one of the functions of which is to protect the feed circuit of the subscriber line of a digital time division multiplex telephone exchange system (German Application 33 12 925), at short intervals of time of equal duration, the current of the subscriber connection line connected to the subscriber telephone circuit in question is scanned and is subjected to analog to digital conversion. The period duration of the scanning here represents a whole-numbered fraction of a period of a longitudinal current produced, for example, by the high voltage currents of a power distribution network. The digitalised scan values are added over this period and the sum value is then compared with a given permissible maximum value. When this maximum value is exceeded, the feed circuit is interrupted.
In this arrangement it is provided that such an interruption is carried out when, even after one single scanning, the value of the maximum permissible current is exceeded by a predetermined extent. No attempt is made to eliminate the influence of longitudinal currents which in this case play a subordinate role.
The analysis of the described measurement and the corresponding initiation of a circuitry measure is effected in the known arrangement by a microprocessor which is assigned, as a sub-control unit, to a group of such subscriber telephone circuits. It has been proved that this type of analysis takes up a substantial proportion of the available processing time of the microprocessor and that the microprocessor is considerably hindered in respect to its proper function as a control unit inasmuch as short-circuits and impermissibly high shunts must be immediately detected and appropriate circuitry measures initiated.
The aim of the invention consists in providing a circuit arrangement for protecting the subscriber feed circuit which is more favorable in this respect and which is also adapted to the special features of the subscriber feed circuit in that it is capable of distinguishing between increased currents due to the charging of capacitors--in particular, high capacity electrolytic capacitors--of the feed circuit and between increased currents caused by short-circuits and undesired shunts.